When an electronic device operates, the energy necessary for the operation is taken in the electronic device. However, it is difficult to consume all the energy taken in for the operation of the electronic device, and part of the energy is consumed without achieving the intended purpose, as thermal energy, for example. It is studied that the consumed energy is reduced to enhance efficiency in the following method. The energy to be consumed is stored once, as electric energy in an electric storage element and the stored energy is reused when necessary.
This method requires an electric storage device capable of supplying the energy necessary for the operation of an electronic device with suitable output. Examples of the electric storage device include a capacitor and a secondary battery. Particularly among them, an electric double layer capacitor having a large capacitance, capable of rapidly charging and discharging, and having high long-term reliability draws attention and is used in many fields.
An electric double layer capacitor has polarizable electrodes mainly made of activated carbon, as a positive electrode and a negative electrode. The withstand voltage of the electric double layer capacitor is 1.2 V when an aqueous electrolytic solution is used, and is 2.5 to 3.3 V when an organic electrolytic solution is used.
However, the energy density of an electric double layer capacitor is smaller than that of a secondary battery. The energy density is proportional to a capacitance and the square of a voltage. Thus, in order to increase the energy density of an electric double layer capacitor, it is necessary to increase at least one of these elements.
In order to increase the voltage of a capacitor, the following proposal is made. By causing the carbon material of the negative electrode to insert (pre-dope) lithium ions, the electric potential of the negative electrode is lowered. Such a capacitor includes the following elements: a negative electrode having inserted lithium ions; a positive electrode as a polarizable electrode; and an electrolytic solution impregnated into the positive electrode and the negative electrode and including a lithium salt. This capacitor is charged and discharged within the range in which lithium ions pre-doped to the negative electrode are not released completely.
FIG. 4A is a top sectional view of a capacitor including lithium ions as cations, shown as an example of a conventional capacitor. FIG. 4B is a partially cutaway front view of electrode wound unit 100 in this capacitor.
As shown in FIG. 4A, this capacitor includes electrode wound unit 100. Electrode wound unit 100 is formed by laminating positive electrode 101 and negative electrode 102 with separator 103 interposed therebetween and winding the laminate in a concentric configuration. As the lithium supply sources, lithium metals (lithium electrodes) 104 and 105 are disposed along the outer periphery and at the center of electrode wound unit 100, respectively. Lithium metal 105 formed at the winding center is supported by rod 109. Rod 109 also serves as an axial rod for supporting electrode wound unit 100. The capacitor is configured so that these elements are housed in case 106 made of aluminum or iron and an electrolytic solution fills the inside.
Each of positive electrode 101 and negative electrode 102 includes a collector (not shown) made of a porous material having holes through the front and back faces. The collector is thus made of a porous material. Therefore, even when lithium metals 104 and 105 are disposed along the outer periphery and at the center of electrode wound unit 100, lithium ions can freely move from lithium metals 104 and 105 between the respective electrodes through the through-holes of the collectors of electrode wound unit 100. As a result, lithium ions can be pre-doped to the whole of negative electrode 102 in advance.
As shown in FIG. 4B, electrode terminals 107 and 108 are connected to positive electrode 101 and negative electrode 102, respectively. Electrode terminals 107 and 108 are led out in the directions opposite to each other along the winding axial direction of cylindrical electrode wound unit 100. Lithium metal 105 formed at the winding center is supported by rod 109. Rod 109 also works as the axial rod for supporting electrode wound unit 100. The outermost circumference of electrode wound unit 100 is fixed by tapes 110 so that the wound shape is maintained. Such a capacitor is disclosed in Patent Literature 1, for example.